The present invention relates to thin film heterojunction solar cells formed from group I-III-VI compound semiconductors such as copper indium diselenide and more particularly to an improved method for making such semiconducting thin films for use in such solar cells.
The background of copper indium diselenide (CIS) cadmium sulfide solar cells is discussed in considerable detail in U.S. Pat. No. 4,335,266, issued to Mickelsen et al on June 15, 1982, which patent is hereby incorporated by reference for all purposes. As generally discussed in this reference, this type of device is considered by many to be a viable solar cell. Numerous improvements in efficiency of such solar cells are well documented by publications and patents covering such improvements.
U.S. Pat. No. 4,611,091, issued to Choudary et al on Sept. 9, 1986, covers one such improvement. This patent is hereby incorporated by reference for its teaching of a structure in which a majority of the cadmium sulfide layer used by Mickelsen is replaced by a substantially conducting wide bandgap n-type semiconductor material such as zinc oxide.
U.S. Pat. No. 4,581,108, issued to Kapur et al on Apr. 8, 1986, is hereby incorporated by reference for its teachings concerning an improved method for forming CIS for use in Mickelsen-type solar cells. In this patent, layers of copper and indium are sequentially deposited on a substrate by electrodeposition. This process allows very close control of the total quantities of these two elements. This patent teaches two alternative methods for formation of CIS. The preferred teaching involves heating of the substrate with copper and indium films at 400 degrees Centigrade in the presence of a selenium-containing gas, preferably H.sub.2 Se. The second method taught is to electrodeposit a separate film of elemental selenium on top of the copper and indium film and then heat the substrate at 400 degrees Centigrade in an inert atmosphere.
The heating of copper and indium films in the presence of a selenium-containing gas has been called selenization. This selenization process has been accepted by a number of researchers in the field as an effective way for forming good quality CIS films for use in the manufacture of heterojunction solar cells. However, a serious problem with the selenization process has been recognized. Hydrogen selenide, H.sub.2 Se, is an extremely toxic gas. It is considerably more toxic than the more commonly encountered gas hydrogen sulfide, H.sub.2 S, which is considered to be quite toxic. Large scale production of CIS-type semiconductor material using the selenization process would necessarily involve the manufacture, storage, and use of large quantities of the H.sub.2 Se gas. This would present a risk of injury or death in the event of an accidental release of the material.
Therefore, research has been conducted on methods of selenization which minimize or eliminate entirely the use of H.sub.2 Se. For example, the use of a solid film of elemental selenium as suggested by Kapur et al does result in production of useful CIS films. However, the quality of such films has been found to be generally inferior to those produced by the preferred selenization process of Kapur et al.